In charge domain pipeline circuits, such as used in Analog to Digital Converters (ADCs) and other applications, signal charge is transferred from a sending capacitive net to a pre-charged receiving capacitive net using a charge transfer device. These capacitive nets are typically implemented as floating diffusions. Transfer of a signal charge into a capacitive net causes the voltage on the capacitive net to drop from a pre-charged voltage by an amount proportional to the amount of charge.
In order to measure the size of the signal charge, as needed for the purpose of comparing it to another charge packet on an opposing floating diffusion of a differential design, it is necessary to either amplify or buffer the change in voltage. This is normally done by connecting the Floating Diffusion (FD) to the gate of a Metal Oxide Semiconductor Field Effect Transistor (MOSFET) which acts as an input device to a buffer circuit.
After the change in voltage has been measured, the received charge packet is sent to the next receiving floating diffusion in the pipeline. This charge packet should be completely transferred without losing any charge to the gate of the buffer/amplifier input device. Unfortunately, this is difficult due to the charging of the parasitic capacitance of these input devices with some of the signal charge. Opposite sides of these parasitic capacitors are connected to internal nets of the amplifier/buffer circuitry and, thus the amount of coupling from these nets back onto the floating diffusion is un-deterministic.